In analyzers of body liquids such as those available from the Eastman Kodak Company under the trademark "Ektachem 700" or "Ektachem 400", it has been conventional to use as part of the analyzer, apparatus for detecting liquid dispensing behavior onto a dried test element, the apparatus comprising an infrared emitting light source, a detector of infrared light, and a support to position a test element relative to the light source and detector. As described for example in U.S. Pat. No. 4,420,566, such light source has been an incandescent lamp and the detector a PbS cell. These were positioned to detect IR radiation REFLECTED from the surface of a test element, inasmuch as the amount reflected changes if water or water-based serum is present. Thus, the apparatus serves as a drop detector, and can be used to detect pre-spotting, an undesirable event that occasionally occurs.
Although such apparatus has been very useful, the lamp has proven to have a relatively short life, and the components are relatively expensive. Known substitutes for the lamp and PbS cell are, respectively, an infrared (IR) light-emitting diode (LED) and a photo-sensitive transistor or photodiode. Further, these are less expensive with a longer life. However, when these are substituted into the analyzer described above for reflection detection, they prove to be too insensitive to be useful. More precisely, they are not capable of detecting from noise, the existence of a pre-spot, a needed function of such a drop detector. This is considered to be due to the wavelengths of light emitted by the LED not being strongly absorbed by water. Thus, this replacement for the drop detector already being used was initially discarded.